1. Field of the Invention
The present invention relates to solid electrolyte films formed on substrates by plasma spraying, solid oxide fuel cells (SOFC) having such films, and processes for producing such films and fuel cells.
2. Related Art Statement
Recently, fuel cells have been noted as power generating devices. The fuel cell is device capable of directly converting chemical energy possessed by fuel to electric energy. Since the fuel cell is free from limitation of Carnot's cycle, the cell is an extremely promising technique in that the fuel cell essentially has a high energy conversion efficiency, a variety of fuels (naphtha, natural gas, methanol, coal reformed gas, heavy oil, etc.) may be used, and the cell provokes less public nuisance, and its power generating efficiency is not influenced by the scale of the equipment.
Particularly, since the solid oxide fuel cell operates at high temperatures of 1,000.degree. C. or more, activity of electrodes is extremely high. Thus, it is not necessary to use catalyst materials of a noble metal such as expensive platinum. In addition, since the SOFC has low polarization and relatively high output voltage, its energy conversion efficiency is conspicuously higher than that in other fuel cells. Furthermore, since their constituent materials are all solid, the SOFC is stable and has long use life.
Although it is desired in the SOFC that the thickness of the solid electrolyte film is made smaller, electrolyte film-forming techniques such as chemical vapor deposition (CVD) or electrochemical vapor deposition (EVD) have problems in that the apparatus becomes large in size, the treated area is too small, and the treating speed is too low.
Plasma spraying has heretofore been used for formation of the solid oxide fuel cells because the plasma spraying has merits in that the film-forming speed is great, the film can be formed simply, thinly and relatively densely (Sunshine 1981, Vol. 2, No. 1).
Further, it is publicly known that after the grain size of a raw material to be plasma sprayed (hereinafter referred to as "plasma spraying raw material"), in which cerium oxide or zirconium oxide is solid solved with an oxide of an alkaline earth metal element or a rare earth element is adjusted, the material is plasma sprayed to form a solid electrolyte film (Japanese patent application Laid-open Nos. 61-198,569 and 61-198,570).
On the other hand, porosity of the solid electrolyte film formed by plasma spraying is generally 5%, and reaches up to 10%, so that such a solid electrolyte film has insufficient density as a solid electrolyte film for SOFC, and cracks or laminar defects occur in the film during the plasma spraying. For this reason, a leakage phenomenon occurs that a fuel such as hydrogen or carbon monoxide penetrates during operation of the SOFC. Consequently, electromotive forces per SOFC is smaller than, for example, 1 V in ordinary fuel cells, so that outputs drop and the conversion rate of the fuel to electric power lowers.
At that time, although it may be considered that the fuel leakage phenomenon is coped with by increasing the thickness of the solid electrolyte film, resistance against diffusion of ions into the bulk in this case becomes greater.
Furthermore, since a number of the laminar defects microstructurally exist as mentioned above, when the conductive solid electrolyte film is formed by plasma spraying, the electric conductivity of the solid electrolyte film is lower than that of a sintered body having the same composition a the solid electrolyte film. Consequently, the internal resistance of the solid oxide fuel cell becomes greater to deteriorate characteristics thereof. Therefore, it is desired that the thickness of the solid electrolyte film itself is decreased to raise outputs of the cell.